Spark ignition device for an internal combustion engine and sparking tip therefor

CROSS-REFERENCE TO RELATED APPLICATION

p-0002This application claims the benefit of U.S. Provisional Application Ser. No. 61/054,215, filed May 19, 2008, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

p-00031. Technical Field

p-0004This invention relates generally to spark ignition devices, such as spark plugs for internal combustion engines, and more particularly to the alloys used to make the sparking surface of the sparking electrode of such devices.

p-00052. Related Art

p-0006Various precious metal alloys are used for the sparking surfaces of spark plug electrodes. Some of the precious metal elements are less costly than others, but there is sometimes a trade-off in terms of the properties that can be achieved using the less costly alloy alternatives. For example, palladium is a relatively low cost element, however it has been found to oxidize in the operating environment of spark plug sparking surfaces. For example, the palladium used in sparking surfaces made from alloys of rhenium-palladium or platinum-palladium-iridium alloys has been shown to oxidizes during use.

p-0007It has been found that alloying gold with palladium has the beneficial effect of mitigating the oxidation of palladium under the operating environment of a spark plug electrode. However, known palladium-gold alloys (e.g., Au40Pd) have mechanical properties believed to be too soft and weak for extended use in modern sparking surface applications. Further, the known palladium-gold alloy materials are prone to alloy with the combustion constituents present in the operating environment, such as lead, thereby causing the material to degrade in use.

SUMMARY OF THE INVENTION

p-0008According to one aspect of the invention, a spark ignition device, such as a spark plug, is provided having an electrode sparking tip made from a AuPd alloy with the addition of a precious metal element of at least 3 wt. % selected from at least one of platinum, iridium, rhodium and ruthenium. The gold counteracts oxidation of the palladium and the additional element or elements provide high temperature mechanical properties and resistance to alloying with the combustion constituents during use.

p-0009According to another aspect of the invention, the Au makes up at least 10 wt. % of the sparking tip.

p-0010According to another aspect of the invention, the AuPd alloy makes up at least 40 wt. % of the sparking tip, with the additional precious metal element making up the difference.

p-0011According to yet another aspect of the invention, the above AuPd and Pt, Ir, Rh and/or Ru alloy can be diluted with a non-precious metal, such as Ni (up to about 30 wt. %), and further include the addition of small amounts of W (1-4 wt. %) for lowering work function, and optionally small amounts of one or more reactive elements, such as Zr, Y, La, Hf, Ta at 0.01-0.2 wt % for grain stabilization.

BRIEF DESCRIPTION OF THE DRAWINGS

p-0012These and other aspects, features and advantages of the invention will become more readily appreciated when considered in connection with the following detailed description of presently preferred embodiments and best mode, appended claims and accompanying drawings, in which:

p-0013<figref idrefs="DRAWINGS">FIG. 1</figref> is a cross-sectional elevation view of an ignition device with an electrode sparking tip constructed in accordance with the invention; and

p-0014<figref idrefs="DRAWINGS">FIG. 2</figref> is an enlarged view of the encircled region <b>2</b> of <figref idrefs="DRAWINGS">FIG. 1</figref>.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

p-0015Referring in more detail to the drawings, <figref idrefs="DRAWINGS">FIG. 1</figref> illustrates a spark ignition device <b>10</b> constructed in accordance with one presently preferred aspect of the invention for use in igniting a fuel/air mixture in internal combustion engines. The exemplary spark ignition device <b>10</b> is illustrated in the form of a spark plug, but the invention contemplates other ignition devices to be within the scope of the claims, such as glow plugs, for example. The device <b>10</b> includes an annular ceramic insulator <b>12</b> fabricated of aluminum oxide or another suitable electrically insulating material in known manner. The insulator <b>12</b> has a central passage <b>14</b> extending longitudinally between an upper terminal end <b>16</b> and a lower nose or core end <b>18</b> in which a center electrode <b>20</b> is disposed. The center electrode <b>20</b> has a sparking surface, referred to hereafter as sparking tip <b>21</b>, at a free end thereof. An electrically conductive metal shell <b>22</b> is disposed in sealed relation about the lower and mid portions of the insulator <b>12</b> and may be made from any suitable metal, such as various steel alloys, and may be coated with a Zn or Ni-base alloy coating or the like in known manner. The shell <b>22</b> includes at least one ground electrode <b>24</b> which may have any of a number of shapes, sizes and configurations, such as the standard single L-shaped configuration illustrated in the drawings, for example. The ground electrode <b>24</b> has at least one ground electrode sparking surface, referred to hereafter as sparking tip <b>25</b>, that is spaced across a spark gap <b>26</b> from the sparking tip <b>21</b> of the center electrode <b>20</b>. At least one of the sparking tips <b>21</b>, <b>25</b> is constructed from AuPd alloy with the addition of at least one of platinum, iridium, rhodium and ruthenium of at least 3 wt. %. The gold counteracts oxidation of the palladium and the additional element adds the needed high temperature mechanical properties and resistance to alloying with the combustion constituents during use.

p-0016An electrically conductive terminal stud <b>28</b> is disposed in the central passage <b>14</b> of the insulator <b>12</b> with a free lower end <b>32</b> of the terminal stud <b>28</b> being disposed adjacent a resistor layer <b>30</b> which is arranged between the lower end <b>32</b> and an upper end <b>34</b> of the center electrode <b>20</b>. Conductive glass seals <b>36</b>, <b>38</b> separate the resistor layer <b>30</b> from the stud <b>28</b> and center electrode <b>20</b>, respectively. This resistor layer <b>30</b> can be made from any suitable composition used in such applications to suppress electromagnetic interference (EMI). The terminal stud <b>28</b>, upper glass seal <b>36</b>, resistor layer <b>30</b>, lower glass seal <b>38</b> and center electrode <b>20</b> provide an electrical path, whereas the shell <b>22</b> and ground electrode <b>24</b> provide a ground path spaced from the electrical path across the spark gap <b>26</b>.

p-0017At least one or both of the sparking tips <b>21</b>, <b>25</b> are constructed of a AuPd-based alloy, whereas the center electrode <b>20</b> and ground electrode <b>24</b> can be fabricated of a material other than that used to construct the sparking tips <b>21</b>, <b>25</b>. If both sparking tips <b>21</b>, <b>25</b> are constructed of a AuPd-based alloy, they may be of the same or different AuPd-based alloy composition. As illustrated, the sparking tips <b>21</b>, <b>25</b> are conductively joined, such as by welding, to the center and ground electrodes <b>20</b>, <b>24</b>, respectively. It should be recognized that the mechanism of attachment of the sparking tips <b>21</b>, <b>25</b> may be performed utilizing any suitable mechanism of attachment, and further, that the shape of the sparking tips <b>21</b>, <b>25</b> may by other than as described or shown, such as disc, cylinders, rivets, bars, or other shapes, as desired.

p-0018The AuPd-based alloy is prealloyed with at least one precious metal effective to increase the high temperature performance properties and increase the mechanical strength of the finished sparking tip material to levels suitable for modern sparking surface applications, as well as to prevent the AuPd constituents from further alloying and breaking down in the presence of common combustion constituents, such as lead. One such precious metal found to be effective in increasing the mechanical strength of the finished sparking tip material and in preventing or inhibiting unwanted alloying is platinum (Pt). Alloys of 55 wt. % Au, 40 wt. % Pd and 5 wt. % Pt; 50 wt. % Au, 30 wt. % Pd and 20 wt. % Pt; and 50 wt. % Au, 40 wt. % Pd and 10 wt. % Pt have proven to be effective as low cost sparking tip materials for spark plugs, while providing the desired mechanical strength and resistance to alloying in use. In general, the Pt can be added in ranges of 3-50 wt. %, 5-20 wt. % or 5-10 wt. % to an alloy of AuPd which is alloyed in a ratio of about 1.5:1, while AuPd alloyed ratios of 0.25:1 to 2:1 are also contemplated herein.

p-0019According to a further aspect of the invention, one or more additional precious metals may be added to the AuPd alloy, such as Ir, Rh and/or Ru either in combination with Pt or in place of Pt. For example, an alloy of 46 wt. % Pt, 34 wt. % Pd, 10 wt. % Au and 10 wt. % Ir has shown to be an effective alloy for sparking tips of spark plugs. According to yet a further aspect of the invention, these alloys can be diluted with non-precious metals, such as Ni and can include small amounts of W (1 to 4 wt. %) for lowering work function. In addition or alternatively, these alloys can be further alloyed with small amounts of one or more reactive elements, such as Zr, Y, La, Hf or Ta in amounts from 0.01-0.2 wt. % as a grain stabilizer for the alloy.

p-0020According to yet another aspect of the invention, the sparking tips <b>21</b>, <b>25</b> can be fabricated of AuPd-based alloys having an Au content greater than 10 wt. % and a Pd content greater than 5 wt. % in combination with at least one of Pt, Ir, Rh or Ru. This material may be further alloyed with small amounts of W as discussed above, as well as optionally being alloyed with one or more reactive elements as discussed above.

p-0021Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. Accordingly, the invention is ultimately defined by the scope of any allowed claims, and not solely by the exemplary embodiments discussed above.